360 in 64 Val?

I just scored a built 360 and am wondering how much HP I would lose if I installed it in my 4speed 64 Valiant using the original exhaust manifolds? It has J heads with the big valves, a custom valley pan, full port and polish, solid lifters and cam, LD340 manifold, etc. etc. Currently there is a very healthy 318 doing the duties in the car and I really enjoy it's pep. BTW I am on a tight to non existent budget. Will this be a largely wasted effort without headers and major exhaust work?

I could put the engine into my 84 D150 using magnum exhaust manifolds but . . . :sad1: I can sort of hear the responses in my head already, but say it anyway.

Thanks!